Supersonic vapor compression and heat rejection cycle

ABSTRACT

Apparatus for cooling a fuel cell stack. The cooling system uses vaporization cooling of the fuel stack and supersonic vapor compression of the vaporized coolant to significantly increase the temperature and pressure of the liquid coolant flowing through a heat exchanger. By increasing the heat rejection temperature of the coolant delivered to the heat exchanger, the heat transfer area of the heat exchanger can be reduced and the mass flow rate of coolant can also be reduced. The increased fluid pressure is used to circulate the coolant through the cooling system, thereby eliminating the circulation pump associated with conventional systems.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus fortransferring heat, and more specifically, for cooling a fuel cell stack.

BACKGROUND OF THE INVENTION

Cooling systems are implemented in a variety of applications for coolinga heat source. Generally, cooling systems include a cooling fluidflowing therethrough, which undergoes phase changes to perform thecooling function. In particular, the cooling fluid cools the heat sourcevia a heat transfer therefrom, whereby the cooling fluid is caused tovaporize from an original liquid form. The coolant fluid, in vapor formflows through a heat exchanger which is in heat exchange communicationwith a lower temperature source, such as ambient air. As the vapor flowsthrough the heat exchanger heat exchange occurs from the vapor, therebypartially transforming the coolant fluid to its liquid phase. Acondenser is also included for condensing the remaining vapor phase tothe liquid phase. A large circulation pump is required for circulatingthe liquid coolant through the heat source and the components of thecooling system.

One such application that requires a cooling system is a fuel cellsystem. Fuel cells have been used as a power source in manyapplications, such as electrical vehicular power plants to replaceinternal combustion engines. In proton exchange membrane (PEM) type fuelcells, hydrogen is supplied to the anode of the fuel cell and oxygen issupplied as the oxidant to the cathode. PEM fuel cells include amembrane electrode assembly (MEA) comprising a thin, protontransmissive, non-electrically conductive, solid polymer electrolytemembrane having the anode catalyst on one face and the cathode catalyston the opposite face. The MEA is sandwiched between a pair ofelectrically conductive elements which (1) serve as current collectorsfor the anode and cathode, and (2) contain appropriate channels and/oropenings formed therein for distributing the fuel cell's gaseousreactants over the surfaces of the respective anode and cathodecatalysts.

The term “fuel cell” is typically used to refer to either a single cellor a plurality of cells (stack) depending on the context. A plurality ofindividual cells are typically bundled together to form a fuel cellstack and are commonly arranged electrically in series. Each cell withinthe stack includes the membrane electrode assembly (MEA) describedearlier, and each such MEA provides its increment of voltage. A group ofadjacent cells within the stack is referred to as a cluster. By way ofexample, some typical arrangements for multiple cells in a stack areshown and described in U.S. Pat. No. 5,763,113.

In PEM fuel cells, hydrogen (H₂) is the anode reactant (i.e., fuel) andoxygen is the cathode reactant (i.e., oxidant). The oxygen can be eithera pure form (O₂) or air (a mixture of O₂ and N₂). The solid polymerelectrolytes are typically made from ion exchange resins such asperfluoronated sulfonic acid. The anode/cathode typically comprisesfinely divided catalytic particles, which are often supported on carbonparticles, and mixed with a proton conductive resin. The catalyticparticles are typically costly precious metal particles. As such theseMEAs are relatively expensive to manufacture and require certainconditions, including proper water management and humidification andcontrol of catalyst fouling constituents such as carbon monoxide (CO),for effective operation.

The electrically conductive elements sandwiching the MEAs may contain anarray of grooves in the faces thereof for distributing the fuel cell'sgaseous reactants (i.e., hydrogen and oxygen in the form of air) overthe surfaces of the respective cathode and anode. In the fuel cellstack, a plurality of cells are stacked together in electrical serieswhile being separated by a gas impermeable, electrically conductive,bipolar plate. The bipolar plate serves several functions including: (a)acting as an electrically conductive gas separator element between twoadjacent cells; (2) distributing reactant gases across substantially theentire surface of the membrane; (3) conducting electrical currentbetween the anode of one cell and the cathode of the next adjacent cellin the stack; (4) keeping the reactant gases separated in order toprevent auto ignition; (5) providing a support structure for the protonexchange membrane; and (6) in most cases, providing internal coolingpassages defined by internal heat exchange faces and through which acoolant flow to remove waste heat from the stack. Various examples of abipolar plate for use in PEM fuel cells are shown and described incommonly-owned U.S. Pat. No. 5,776,624.

Current fuel cell cooling systems are undesirably large, including thelarge circulation pump for circulating the liquid coolant through thefuel cell stack (i.e. heat source) to the heat exchanger where the wastethermal energy (i.e., heat) is transferred to the environment. Thethermal properties of typical liquid coolants require a large volume tobe circulated through the system to reject sufficient waste heat tomaintain the stack operating temperature, particularly under maximumpower conditions. For example, a PEM fuel cell stack operating at 80 KWand 50% efficiency with an operating temperature of 80° C. will generate80 KW of waste heat that must be rejected. However, since a maximumambient air temperature of about 40° C. can be utilized for heatrejection, a mass flow rate of approximately 2000-3000 grams/sec. ofcoolant must flow through the stack in combination with use of largeheat exchanger areas to accommodate the required heat rejection. As iswell known, the expense associated with large heat exchangers and theother cooling system components (recirculation pump, proportional mixingvalves, PID controllers, etc.), combined with packaging constraintscaused by physical size requirements of the components, have had adetrimental impact on widespread commercialization of fuel cell systems.Thus, a need exists to develop alternative fuel cell cooling systemswhich overcome the shortcomings of conventional cooling systems andassist in advancing the art.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for extractingwaste heat from a heat source, such as a fuel cell stack, and rejectingthe waste heat to the environment through a heat exchanger.

In one aspect, the method of the present invention extracts waste heatfrom a heat source, such as a fuel cell stack, by circulating a heattransfer fluid in a flow path between the fuel cell stack and the heatexchanger, and transferring heat from the fuel cell stack to the fluidby causing a portion of the fluid to vaporize. The energy required tovaporize the liquid coolant is significantly greater than the heatcarrying capacity of the liquid.

In another aspect of the method of the present invention, the waste heatis rejected through the heat exchanger by separating the coolantdischarged from the fuel cell stack into a vapor stream and a liquidstream, accelerating the vapor stream to supersonic speed, contactingthe high velocity vapor stream with a portion of the liquid stream, andtransferring the vapor stream momentum to the liquid. The resulting hightemperature, high pressure liquid stream is delivered to the heatexchanger for heat rejection to the environment.

The apparatus of the present invention comprises a vapor separator forseparating the coolant exiting the fuel cell stack into a liquid streamand a vapor stream, and a supersonic nozzle/ejector unit having a vaporinlet receiving the vapor stream and a liquid inlet receiving the liquidstream. The nozzle/ejector unit is operable to accelerate the vaporstream to supersonic velocity. The supersonic vapor stream acceleratesthe liquid stream as both travel toward the outlet of the nozzle/ejectorunit. The high velocity vapor/liquid mixture enters the outlet of thenozzle/ejector unit which condenses the vapor and causes an increase inthe coolant pressure and temperature. The high temperature, highpressure coolant is then delivered to the first heat exchanger where thefluid temperature is reduced to the operating temperature of the fuelcell stack. The fluid is then circulated to the inlet of the fuel cellstack. As such, the nozzle/ejector unit generates sufficient fluidpressure to drive circulation of the coolant through the entire systemand further increases the heat rejection temperature of the fluiddelivered to the heat exchanger for permitting reductions in the heatexchanger surface area and the mass flow rate of the coolant.

In accordance with one aspect of the present invention, the liquidstream from the vapor separator is returned to the inlet of the fuelcell stack and the liquid stream delivered to the liquid inlet of thenozzle/ejector unit is routed from a portion of the coolant flowdownstream of the heat exchanger. As a related aspect, a second heatexchanger is disposed between the first heat exchanger and the liquidinlet of the nozzle/ejector unit for cooling the liquid stream prior todelivery to the nozzle/ejector unit. This arrangement is operable toestablish a desired temperature difference between the vapor stream andthe liquid stream delivered to the inlet side of the nozzle/ejectorunit.

The present invention utilizes vapor cooling of a heat source, such asthe fuel cell stack, to provide improved control over the stackoperating temperature. In addition, the enthalpy of the vapor is used toraise the heat rejection temperature of the coolant delivered to theprimary heat exchanger and its fluid pressure is used to circulate thecoolant. This permits elimination of the large circulation pump used inconventional cooling systems which significantly improves the overallefficiency of the fuel cell system. The invention is also adaptable foruse in a variety of systems where heat transfer occurs, and it isdesired to decrease pumping power and increase the temperature at whichheat is rejected.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features, advantages and other uses of the present inventionwill become more apparent by referring to the following description anddrawings in which:

FIG. 1 is a schematic diagram of a cooling system according to theprinciples of the present invention;

FIG. 2 is a schematic view of an alternative cooling system inaccordance with the principles of the present invention;

FIG. 3 is a schematic view of a second alternative cooling system inaccordance with the principles of the present invention;

FIG. 4 is a sectional view of a supersonic nozzle/ejector unitassociated with each of the cooling systems shown in FIGS. 1 through 3;

FIG. 5 is a sectional view of an alternative embodiment of thesupersonic nozzle/ejector unit;

FIG. 6 is an exploded isometric view of a PEM fuel cell stack;

FIG. 7 is a diagram illustrating a particular use application for thefuel cell stack of FIG. 6;

FIG. 8 is a schematic view of a third alternative cooling system inaccordance with the principles of the present invention; and

FIG. 9 is a schematic view of the third alternative cooling systemconfigured for heating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a cooling system 10 is provided for cooling aheat source 12. The cooling system 10 includes a supersonicnozzle/ejector unit 11, a primary heat exchanger 14, a secondary heatexchanger 16, a splitter valve 18, a pressure regulator valve 20, amixer 22, a vapor separator 24 and a pump 26. The various components ofthe cooling system 10 are configured in a circuit for providing fluidcommunication therebetween. In particular, cooling fluid circulatingthrough the cooling system 10 is in heat exchange relationship with theheat source 12 for cooling the heat source 12. As described in furtherdetail herein, the cooling fluid, having cooled the heat source 12, isheated to a partial vapor, liquid state. The vapor separator 24separates the vapor fraction from the liquid fraction as the coolantexits the heat source 12. The liquid fraction is pumped by the pump 26back around to the mixer 22 for further cooling of the heat source 12.The vapor fraction is directed to the nozzle/ejector unit 11.

The nozzle/ejector unit 11 utilizes the vapor fraction discharged fromthe heat source 12 to increase the temperature and pressure of coolantfluid supplied to the primary heat exchanger 14, as described in furtherdetail herein. The higher temperature and pressure coolant fluiddischarged from the nozzle/ejector unit 11 flows through the primaryheat exchanger 14 where heat transfer to ambient occurs, therebyreducing the temperature and pressure of the coolant fluid. The splittervalve 18 splits the stream exiting the primary heat exchanger 14 into afirst liquid stream supplied to the secondary heat exchanger 16 and asecond liquid stream routed toward the heat source 12. The splittervalve 18 also reduces the pressure of the second liquid stream. Thesecondary heat exchanger 16 functions to reduce the temperature of theliquid coolant delivered to the liquid inlet of the nozzle/ejector unit11 to a value below the vaporization temperature of the coolant.

The pressure regulator valve 20 functions to reduce the pressure of thehigh pressure liquid coolant discharged from the mixer 22 to the heatsource inlet pressure for mixing with the liquid fraction in the mixer22. The mixer 22 combines the liquid coolant flowing from the primaryheat exchanger 14 with the liquid coolant from the vapor separator 24.The small low-power return pump 26 delivers the coolant recycled fromvapor separator 24 to the mixer 22. The outlet of the mixer 22 isdelivered to the heat source 12. The pump 26 can also be used duringstart-up of the cooling system 10.

In its most basic form, the cooling system 10 incorporates the use ofvaporization cooling of the heat source 12 and supersonic vaporcompression of the vaporized coolant to provide significant advantagesover conventional liquid coolant systems. In particular, the energyrequired to vaporize a liquid coolant as it flows through the coolingplates of a high temperature heat source is much greater than the heatcarrying capacity of the liquid coolant. As a result, the mass flow rateof coolant required for stack cooling and which is circulated throughcooling system is significantly reduced. With regard to supersonic vaporcompression, the enthalpy of the vapor discharged from the heat source12 is utilized to increase the temperature and pressure of the coolantdelivered to a primary heat exchanger 14. Specifically, a subsonicstream of coolant vapor is delivered to the gas inlet of thenozzle/ejector unit 11 and a stream of liquid coolant is delivered tothe liquid inlet of the nozzle/ejector unit 11. The stream of coolantvapor is expanded as it flows through a nozzle section of thenozzle/ejector unit 11 to generate a supersonic vapor stream due toconversion of heat energy into kinetic energy. The supersonic vaporstream entrains the subsonic liquid stream in an ejector section of thenozzle/ejector unit 11. As the vapor/liquid mixture reaches a dischargesection of the nozzle/ejector unit 11, the pressure rises which acts tocondense the vapor and significantly increase the temperature of theliquid coolant. This high temperature, high pressure stream of liquidcoolant is delivered to the primary heat exchanger 14 where rejection ofwaste heat to a low temperature heat sink (i.e., ambient air) causes theliquid temperature to be reduced to the operating temperature of theheat source 12.

As a result of utilizing vapor compression, the heat rejectiontemperature of the liquid coolant delivered to the primary heatexchanger 14 is significantly increased, thereby permitting acommensurate reduction in size (i.e., surface area) of the primary heatexchanger 14 compared to the large heat exchangers associated withconventional liquid cooled systems. In addition, the increased liquidpressure discharged from the nozzle/ejector unit 11 circulates thecoolant through the entire system 10. As such, the cooling system 10requires little, if any, input energy to run and eliminates the largerecirculation pump required in conventional cooling systems, therebyincreasing the overall efficiency. While alternative embodiments of thecooling system 10 will be described hereinafter, each results insignificant advantages over conventional systems. These advantagesinclude, among others, reduced mass flow rates and heat exchanger sizes,elimination of the coolant recirculation pump and its system loadrequirements, improved packaging opportunities, and improved control ofthe heat source 12 operating temperature while concomitantly producingimproved heat transfer capability.

With particular reference to FIG. 2, an alternative cooling system 30 isprovided, functioning by the same principles as the cooling system 10 ofFIG. 1. In particular, the cooling system 30 utilizes a coolant fluidthat can be a mixture comprised of a predetermined mixture of ammonia(NH₃) and water (H₂O). The cooling system 30 includes the nozzle/ejectorunit 11, a primary heat exchanger 34, a secondary heat exchanger 36, ahydraulic motor/pump 38, a pressure regulator valve 40, and a vaporseparator 42. The various components of the cooling system 30 areconfigured in a circuit for providing fluid communication therebetween.In particular, cooling fluid circulating through the cooling system 30is in heat exchange relationship with the heat source 12 for cooling theheat source 12. As described in further detail herein, the coolingfluid, having cooled the heat source 12, is heated to a partial vapor,liquid state. The vapor separator 42 separates the vapor fraction fromthe liquid fraction as the coolant exits the heat source 12. The liquidfraction flows through the secondary heat exchanger 36, reducing thetemperature and pressure thereof and is then pumped by the hydraulicmotor/pump 38 to an increased pressure and directed to the liquid inletof the nozzle/ejector unit 11. The vapor fraction is directed to thevapor inlet of the nozzle/ejector unit 11.

The nozzle/ejector unit 11 utilizes the vapor fraction discharged fromthe heat source 12 to increase the temperature and pressure of liquidsupplied to the primary heat exchanger 34, as described above. Thehigher temperature and pressure liquid from the nozzle/ejector unit 11flows through the primary heat exchanger 34 where heat transfer toambient occurs, thereby reducing the temperature and pressure of theliquid. In one preferred embodiment, the high pressure liquid isutilized to provide power for the hydraulic motor/pump 38, and theliquid is fed to the heat source 12 at a reduced pressure.Alternatively, however, the pressure regulator valve 40 acts to reducethe liquid discharged from the primary heat exchanger 34 to the heatsource inlet pressure.

With reference to FIG. 3, a second alternative cooling system 50 isshown. The cooling system 50 includes the nozzle/ejector unit 11, aprimary heat exchanger 52, a secondary heat exchanger 54, a firstpressure regulator valve 56, a second pressure regulator valve 58 and avapor separator 60. The various components of the cooling system 50 areconfigured in a circuit for providing fluid communication therebetween.In particular, coolant circulating through the cooling system 50 is inheat exchange relationship with the heat source 12 for cooling the heatsource 12. As described in further detail herein, the coolant, havingcooled the heat source 12, is heated to a partial vapor, liquid state.The vapor separator 60 separates the vapor fraction from the liquidfraction as the coolant exits the heat source 12. The liquid fractionflows to the liquid inlet of the nozzle/ejector unit 11 and the vaporfraction is directed to the vapor inlet of the nozzle/ejector unit 11.

The nozzle/ejector unit 11 utilizes the vapor fraction discharged fromthe heat source 12 to increase the temperature and pressure of liquidsupplied initially to the secondary heat exchanger 54 and ultimately tothe primary heat exchanger 52, as described above. The highertemperature and pressure liquid from the nozzle/ejector unit 11 flowsthrough the secondary heat exchanger 54 where heat transfer to the vaporfraction fed to the vapor inlet of the nozzle/ejector unit 11 occurs.The liquid discharged from the secondary heat exchanger 54 flows at areduced temperature and pressure to the first pressure regulator valve56, whereby its pressure is reduced prior to entry into the primary heatexchanger 52. Heat transfer to ambient occurs as the coolant flowsthrough the primary heat exchanger 52, thereby reducing the temperatureand pressure of the liquid. The liquid then flows through the secondpressure regulator valve 58 which reduces the pressure of the liquid tothe heat source inlet pressure.

The cooling system 50 is a modified version of the cooling system 30 ofFIG. 2 with the secondary heat exchanger 54 positioned such that heatextracted therefrom is used to increase the temperature of the vaporinlet stream sufficiently to establish the required temperature gradientbetween the vapor and liquid delivered to the inlet of nozzle/ejectorunit 11.

Referring now to FIG. 4, a first embodiment of nozzle/ejector unit 11 isshown in section to include a valve body 200 and a nozzle block 202mounted in valve body 200. Valve body 200 is a hollow cylindricalstructure which defines a nozzle section 204, a converging ejectorsection 206, and a discharge section 208. An annular vapor inlet chamber210 is defined between a portion of nozzle block 202 and a valve body200. The subsonic vapor inlet stream flowing from the discharge side ofthe fuel cell stack is delivered to inlet chamber 210 through one ormore vapor inlet ports 212. Likewise, the subsonic liquid inlet streamis delivered through a central liquid inlet port 216 to one end of along liquid flow passage 214 formed in nozzle block 202.

Nozzle section 204 of valve body 200 defines an annular vapor nozzle 218defined by a restricted throat area formed between aconvergent/divergent inner wall surface of valve body 200 and afrusto-conical outer wall surface of nozzle block 202. An expansionchamber 220 is located downstream of vapor nozzle 218 and upstream of aliquid nozzle 222 formed at the terminal end of liquid flow passage 214.In addition, an acceleration chamber 224 is formed by ejector section206 of valve body 200 downstream of expansion chamber 220. Ejectorsection 206 converges to define a discharge flow passage 226 at thedownstream end of acceleration chamber 224. Finally, discharge section208 of valve body 200 includes divergent wall surfaces defining adiffuser chamber 228 which terminates in an outlet port 230 throughwhich the high temperature, high pressure liquid is discharged fromnozzle/ejector unit 11.

In operation, the liquid inlet stream flowing through flow passage 214and discharged through liquid nozzle 222 forms a free liquid jet 234that extends through acceleration chamber 224 to discharge passage 226where it subsequently enters diffuser chamber 228. Liquid jet 234 flowsthrough acceleration chamber 224 without contacting the converging innerwall surfaces of ejector section 206. The area of liquid nozzle 222defines the size and flow rate of liquid jet 234 based on the pressuregradient between the higher fluid pressure of the liquid inlet streamand the vapor pressure in acceleration chamber 224. The subsonic vaporinlet stream supplied to inlet chamber 210 is directed to vapor nozzle218 which, in turn, directs the condensable vapor into expansion chamber220. The vapor pressure of the subsonic vapor inlet stream forces thevapor stream at sonic velocity through vapor nozzle 218. Thereafter, theexpanding vapor is further accelerated in expansion chamber 220 tosupersonic velocity prior to entering acceleration chamber 224. The highvelocity vapor surrounds and impinges upon liquid jet 234 so as totransfer the kinetic energy of the vapor to the liquid, therebyaccelerating the liquid to a higher velocity and ultimately yielding ahigher output pressure.

In acceleration chamber 224, the supersonic vapor impinges on andaccelerates liquid jet 234 as it travels toward discharge passage 226.As a result of this transfer of kinetic energy to liquid jet 234, thevapor condenses as it travels the length of acceleration chamber 224.This results in a transfer of heat to the liquid for significantlyincreasing the temperature of liquid jet 234. As the vapor/liquidmixture enters discharge passage 226, the pressure rises furthercondensing the vapor and increasing the liquid temperature. The coolantdischarged through discharge passage 226 to diffuser chamber 228 issubstantially all liquid which is desired for transferring the liquid'skinetic energy into an amplified output pressure. Thus, kinematic vaporcompression occurring within nozzle/ejector unit 86 involves convertingvapor energy to a high velocity flow and transferring the kinetic energyto a slower moving liquid stream flowing unrestricted in a free jet soas to establish a high pressure, high temperature subsonic liquid outletstream.

Referring now to FIG. 5, a second embodiment of supersonicnozzle/ejector unit 11 is shown to include a hollow cylindrical valvebody 300 and a nozzle tube 302 disposed within valve body 300. Valvebody 300 defines an inlet nozzle section 304, an ejector section 306,and a discharge section 308. An annular vapor inlet chamber 310 isdefined between valve body 300 and nozzle tube 302 and is adapted toreceive the subsonic vapor inlet stream. Likewise, the subsonic liquidinlet stream is delivered through an elongated flow passage 314 formedin nozzle tube 302. Nozzle section 304 of valve body 300 defines anannular vapor nozzle 318 that is formed between a convergent/divergentsection of valve body 300 and nozzle tube 302. An expansion chamber 320is located downstream of vapor nozzle 318 and upstream of a liquidnozzle 322 formed at the terminal end of liquid flow passage 314. Inaddition, an acceleration chamber 324 is formed between nozzle tube 302and a bulbous portion 325 of ejector section 306. Discharge section 308of valve body 300 includes a divergent portion 327 and a diffuserportion 329, both located downstream of liquid nozzle 322.

In a manner substantially similar to operation of the nozzle/ejectorunit shown in FIG. 4, the nozzle/ejector unit shown in FIG. 5 creates asupersonic vapor stream in expansion chamber 320 as subsonic vapor isforced through vapor nozzle 318. The high velocity vapor impinges theliquid jet (not shown) discharged from liquid nozzle 322 so as toaccelerate the liquid stream. The transfer of kinetic energy from thevapor to the liquid causes the vapor to condense so as to furthertransfer heat to the liquid for generating a significant temperatureincrease. A pressure increase occurs as the vapor/liquid mixture entersa restricted discharge passage 326 associated with discharge section308.

Each of the cooling systems 10, 30, 50 described herein may beimplemented in various applications. For example, the cooling systems10, 30, 50 may be implemented to cool a heat source of a vehicleapplication, such as a fuel cell system. Alternatively, the coolingsystems 10, 30, 50 can be implemented as air conditioning systems (i.e.heating and cooling) for a structure such as a building. It will beappreciated, however, that the cooling systems 10, 30, 50 of the presentinvention are not limited to implementation in the exemplaryapplications described herein. The particular function of each of thecooling systems 10, 30, 50 implemented in each of these exemplaryapplications will be discussed in detail.

Before describing implementation of the cooling systems 10, 30, 50 in afuel cell system, it is useful to understand an exemplary fuel cellsystem. Specifically, the fuel cell system shown in FIG. 6 is atwo-cell, bipolar proton exchange membrane (PEM) type fuel cell stack300 having a pair of membrane electrode assemblies (MEAs) 304 and 306separated from each other by an electrically conductive, bipolar plate308. MEAs 304, 306 and bipolar plate 308 are stacked together betweenstainless steel clamping plates 310, 312 and end contact elements 314,316. End contact elements 314, 316, as well as bipolar plate 308,contain a plurality of grooves and openings 318, 320, 322, and 324 fordistributing fuel and oxidant gases (i.e., H₂ and O₂) to MEAs 304 and306. Nonconductive gaskets 326, 328, 330 and 332 provide seals andelectrical insulation between the several components of the fuel stack300. Connectors (not shown) are attached to clamping plates 10 and 12 toprovide positive and negative terminals for the fuel cell stack 300.

With continued reference to FIG. 6, gas permeable carbon/graphitediffusion papers 334, 336, 338, and 340 are shown to be arranged topress against the electrode faces of MEAs 304 and 306. In addition, endcontact elements 314 and 316 press against the carbon/graphite papers334 and 340, respectively, while bipolar plate 308 presses againstcarbon/graphite paper 336 on the anode face of MEA 304 and againstcarbon/graphite paper 338 on the cathode face of MEA 306. Oxygen issupplied to the cathode side of the fuel cell stack from a storage tank346 through the appropriate supply plumbing 342. In addition, hydrogenis supplied to the anode side of the fuel cell stack from a storage tank348 via appropriate supply plumbing 344. Alternatively, air may besupplied to the cathode side from the ambient and hydrogen to the anodefrom a methanol reformer or the like. Exhaust plumbing for both the H₂and O₂/air sides of the MEAs, while not shown, is also provided.Additional plumbing 350, 352 and 354 is provided for supplying coolantfrom an inlet header (not shown) of the fuel cell stack to bipolar plate308 and end plates 314 and 316. Similar plumbing for exhausting coolantfrom bipolar plate 308 and end plates 314 and 316 to an exhaust headerof the fuel cell stack is also provided, but not shown. As will bedetailed, the cooling systems 10, 30, 50, constructed according to thevarious embodiments of the present invention, connect between thestack's inlet and exhaust headers and is operable to remove waste heatfrom the fuel cell stack 300 for rejection to the environment.

The fuel cell stack 300 shown is fueled by an H₂-rich reformateregardless of the method by which such reformate is made. It is to beunderstood that the principles embodied herein are applicable to fuelcells fueled by H₂ obtained from any source, including reformablehydrocarbon and hydrogen-containing fuels, such as methanol, ethanol,gasoline, alkene, or other aliphatic or aromatic hydrocarbons, or fromfuel stored on board, such as H₂.

FIG. 7 shows a preferred embodiment for the fuel cell system utilizingthe fuel cell stack 300, constructed as shown in FIG. 6, in conjunctionwith a vehicle propulsion system 400. Propulsion system 400 is shown toinclude a battery 402, an electric motor 404, and its associated driveelectronics including an inverter 406. Inverter 406 accepts electricenergy from a DC/DC converter 408 associated with fuel cell system, andparticularly from fuel cell stack 300, and to convert the electricalenergy to mechanical energy produced by motor 410. Battery 402 isconstructed and arranged to accept and store electrical energy suppliedby fuel cell stack 300 and to accept and store electrical energysupplied by motor 410 during regenerative braking, and to provideelectric energy to motor 410. Motor 410 is coupled to a driving axle 412to supply motive rotary power to the wheels of a vehicle (not shown). Anelectrochemical engine control module (EECM) 414 and a battery packmodule (BPM 416) monitor various operating parameters, including, butnot limited to, the voltage and current of the fuel cell stack 300. Forexample, this is done by BPM 416, or by 416 and 414 together, to send anoutput signal (message) to a vehicle controller 418 based on conditionsmonitored by BPM 416. Vehicle controller 418 controls actuation ofelectric motor 410, the drive electronics including inverter 406, DC/DCconverter 408, and requests a power level from EECM 414.

Controller 418 may comprise any suitable microprocessor,microcontroller, personal computer, etc., which has central processingunit capable of executing a control program and data stored in a memory.When activated, controller 418 carries out a series of operations storedin an instruction-by instruction format in memory for providing enginecontrol, diagnostic and maintenance operations. Controller 418 may be adedicated controller specific to the present invention or implemented insoftware stored in the main vehicle electronic control module. Further,although software based control programs are usable for controllingsystem components in various modes of operation as described above, itwill also be understood that the control can also be implemented in partor whole by dedicated electronic circuitry.

Referring again to FIG. 1, the cooling system 10 is shown having acoolant, such as methanol, circulated through a closed-loop system forremoving waste heat from the heat source (i.e., the fuel cell stack 300)and rejecting the waste heat primarily through the primary heatexchanger 14 to the environment. With reference to Table A providedbelow, various characteristics of the methanol coolant at various flowposition the cooling system 10 will be disclosed in accordance with anexemplary stack operating configuration (i.e., a 80 kW PEM stackoperating at 80° C.). Each flow position is identified by a positionblock having a corresponding reference numeral assigned thereto.

TABLE A FIG. 1 POSITION FLOW RATE BLOCK TEMP (° C.) PRESSURE (kPa)(gm/sec) 110 80 190 93.5 112 80 181 93.5 114 80 181 74.8 116 80 181 18.7118 57 1158 523.8 120 105 1283 598.6 122 80 1219 598.6 124 80 1219 523.8126 80 1219 598.6 128 80 190 74.8 130 80 190 18.7

At position block 110, a stream of liquid methanol is delivered to theinlet header of the stack 300. As the liquid methanol flows through thecooling passages within the stack 300, it partially vaporizes and exitsthe exhaust header of the stack 300 with the characteristics listed forposition block 112. To cause such vaporization, the outlet pressure ofthe liquid/vapor mixture exiting the stack 300 is selected andmaintained such that the vaporization temperature of the coolant isequal to the stack's operating temperature. For methanol, an outletpressure of about 181 kPa equates to a stack operating temperature of80° C. Moreover, the vaporization mass flow rate for methanol is about74.8 gm/sec. Assuming a 25% excess flow is required to assure adequatecooling across the entire stack during maximum power conditions, a totalcirculation through the heat source 12 of about 93.5 gm/sec is utilizedin the cooling system 10.

Position block 114 represents the characteristics of the vapor fractionand position block 116 represents the characteristics of the liquidfraction downstream of the vapor separator 24. As described in greaterdetail above, vapor compression within the nozzle/ejector unit 11 causesthe liquid stream discharged from the discharge section of thenozzle/ejector unit 11 to be significantly higher in temperature andpressure than either of the inlet vapor and liquid streams alone orcombined. To this end, position block 120 identifies the characteristicsof the subsonic condensed liquid coolant discharged from thenozzle/ejector unit 11.

Assuming a maximum ambient temperature of 40° C. flowing across the heattransfer surface areas of the primary heat exchanger 14, the 105° C.heat rejection temperature of the liquid methanol coolant flowingthrough the primary heat exchanger 14 produces a 65° C. temperaturedifference that is used for efficiently transferring waste heat to theenvironment. Compared to a 40° C. temperature difference associated witha conventional liquid cooling system (80° C. liquid subtracted from 40°C. ambient air), the 65° C. temperature difference produced by vaporcompression within the nozzle/ejector 11 unit permits a proportionalreduction in the heat transfer surface areas required from the primaryheat exchanger 14. That is, a reduction corresponding to the 25° C.temperature difference. The high pressure liquid stream exiting theprimary heat exchanger 14 is at 80° C. which is the operatingtemperature of the stack 300. The small pressure drop across the primaryheat exchanger 14 is attributable to line losses. Position block 122identifies the liquid characteristics downstream of the primary heatexchanger 14.

The liquid is then split within the splitter valve 18, resulting inapproximately 87.5% of the liquid flow to be fed to the secondary heatexchanger 16 and the remaining 12.5% to be fed back toward the stack300. Characteristics of the liquid portion flowing to the secondary heatexchanger 16 are identified by position block 124 and characteristics ofthe liquid portion flowing back to the stack 300 are identified byposition block 126. Flowing through the secondary heat exchanger 16, theliquid portion experiences a heat transfer and thus, a temperature dropof approximately 23° C., to 57° C. as shown by position block 118. Inthis manner, the temperature of the liquid portion is appropriate foroptimal operation of the nozzle/ejector unit 11. Specifically, optimaloperation of the nozzle/ejector unit 11 requires that the temperature ofthe subsonic liquid stream be less than the temperature of the subsonicvapor stream.

The pressure regulator valve 20 reduces the pressure of the secondliquid stream to the coolant inlet pressure of 190 kPa. Position block128 sets forth the liquid characteristics downstream of the pressureregulator valve 20. After experiencing a pressure reduction through thepressure regulator valve 20, the second liquid stream flows to the mixer22 for mixing with the liquid stream flowing from the vapor separator24. The liquid stream from the vapor separator 24 is initially at atemperature of 80° C. and a pressure of 181 kPa, as identified byposition block 116. The liquid stream is pumped back to the mixer 22 bythe pump 26, thereby experiencing a pressure increase to 190 kPa, asindicated by position block 130. The liquid streams are mixed within themixer 22 and flow to the inlet header of the stack for cooling thereof.

The cooling system 10 detailed in reference to FIG. 1, disclosed to usemethanol as the coolant. However, it is contemplated that any two phaseworking fluid having a vaporization temperature and pressure within therange of the operating characteristics of the fuel cell system can beused in substitution for methanol. Moreover, the present invention alsocontemplates use of two component coolants for use with the vaporizationand supersonic vapor compression features to cool the stack.Specifically, the cooling system 30 of FIG. 2 utilizes a coolant mixturecomprised of a predetermined mixture of ammonia (NH₃) and water (H₂O).Table B sets forth the characteristics of the two component coolant atspecific positions in the closed-loop recirculatory cooling system.

TABLE B FIG. 2 Position Liquid Mass Fraction Vapor Mass Fraction TempPressure Block NH₃ H₂O NH₃ H₂O (° C.) (kPa) 140 71% 29% 77 2536 142 65%35% 99.4% 0.6% 80 2479 144 99.4% 0.6% 80 5500 146 65% 35% 80 2479 14865% 35% 61 2355 150 65% 35% 61 5000 152 71% 29% 107 5500 154 71% 29% 775225 156 71% 29% 77 5225

Referring still to FIG. 2, the coolant enters the stack 300 at its inletheader with a NH₃/H₂O ratio of 71/29 on a percentage mass basis at thetemperature, pressure and flow rate set forth in Table B for positionblock 140. Vaporization of the mixed coolant as it flows through thestack 300 results in differing ratios for the vapor fraction and theliquid fraction, at position block 142 as separated by the vaporseparator 42, as denoted by position blocks 144 and 146, respectively.The vapor fraction is delivered to the vapor inlet of the nozzle/ejectorunit 11 at 80° C. The liquid fraction is transferred through a thesecondary heat exchanger 36 for cooling prior to delivery to the liquidinlet of nozzle/ejector unit 11. Position block 148 indicates atemperature reduction from 80° C. to 61° C. generated by extracting heatfrom the secondary heat exchanger 36. A minimal pressure drop of about124 kPa across the secondary heat exchanger 36 is due to line losses.The liquid fraction is then pumped through the hydraulic motor/pump 38,thereby experiencing a significant increase in pressure, as indicated atposition block 150, prior to flow into the nozzle/ejector unit 11.

As a result of vapor compression within the nozzle/ejector unit 11, theresulting liquid outlet stream has a significantly increased temperatureand pressure, as indicated by the characteristic values noted in Table Bfor position block 152. It should also be noted that the vaporcompression results in re-establishment of the original mixture ratio.The outlet liquid stream flows into the primary heat exchanger 34,whereby the temperature of the liquid coolant flowing through theprimary heat exchanger 34 is reduced from 107° C. to 77° C. (the desiredoperating temperature of the stack) as shown by position block 154. Theprimary heat exchanger 34 is again used to reject a substantial portionof the waste heat to the environment. The liquid coolant flowing fromthe primary heat exchanger 34 either flows through the hydraulicmotor/pump 38 or the pressure regulator valve 40, depending upon thesystem configuration, experiencing a pressure decrease of approximately2688 kPa therethrough and flows back into the stack 300 for coolingthereof.

The second alternative cooling system 50 of FIG. 3 also utilizes acoolant mixture comprised of a predetermined mixture of ammonia (NH₃)and water (H₂O). Table C sets forth the characteristics of the twocomponent coolant at specific positions in the closed-loop recirculatorycooling system 50.

TABLE C FIG. 3 POSITION % NH₃ TEMP PRESSURE FLOW RATE BLOCK % H₂O (° C.)kPa (gm/sec) 160 59/41 67 280 68 162 78/22 80 275 48 164 78/22 90 275 48166 15/85 80 270 20 168 59/41 162 8273 68 170 59/41 150 8273 68 17259/41 150 1000 68 174 59/41 67 1000 68

The circulation of the coolant through the cooling system 50 is similarto that described above for the cooling systems 10, 30. Therefore, adetailed description of the coolant circulation of the cooling system isforegone. The cooling system 50 of FIG. 3 is configured, whereby heatextracted from the secondary heat exchanger 54 is used to heat the vaporfraction supplied to the vapor inlet of the nozzle/ejector unit 11. Inthis manner, the required temperature gradient between the vapor andliquid delivered to the respective inlets of the nozzle/ejector unit 11is maintained.

In one aspect, the cooling systems 10, 30, 50 described herein, arefunctional to cause a heat transfer from an intermediate temperature toa higher temperature in combination with a lower temperature heat sink.Traditional cooling systems require a significant amount of work andadditional components to achieve a similar result. Tables A, B and Cdetail operation of the cooling systems 10, 30, 50, respectively, for afuel cell stack cooling application, whereby the heat source 12 is thehigh temperature source (approximately 80° C.), the primary heatexchanger 14, 34, 52 is in heat exchange relationship with theintermediate temperature source (approximately 40° C.) and the secondaryheat exchanger 16, 36, 54 is in heat exchange relationship with the lowtemperature source (approximately 25° C.).

As discussed above, the cooling systems 10, 30, 50 are applicable inother applications, such as air conditioning (heating and cooling) of astructure (e.g. building, house and the like). In such an application,parallel cooling systems would be required, the first to perform thecooling function and the second to perform the heating function, withthe object to be temperature control of the structure. It will beappreciated, however, that although parallel systems are required, asignificant advantage is maintained in that the cooling systems haveminimal energy requirements to function. Thus, in a power outagesituation, the cooling systems can continue effectively operating usingbattery power or the like. With reference to FIGS. 8 and 9, a coolingsystem 500 is provided respectively configured for cooling and heating astructure 510.

With particular reference to FIG. 8, the cooling system 500 includes asupersonic nozzle/ejector unit 512, a primary heat exchanger 514, asecondary heat exchanger 516, a splitter valve 518, a pressure regulatorvalve 520, a mixer 522, a vapor separator 524 a first pump 526 and asecond pump 528. The various components of the cooling system 500 areconfigured in a circuit for providing fluid communication therebetween.In particular, cooling fluid circulating through the cooling system 500is in heat exchange relationship with the structure 510 for cooling thestructure 510. As described in further detail herein, the cooling fluid,having cooled the structure 510, is heated to a partial vapor, liquidstate. The vapor separator 524 separates the vapor fraction from theliquid fraction as the coolant exits the structure 510. The liquidfraction is pumped by the first pump 526 back around to the mixer 522for further cooling of the structure 510. The vapor fraction is directedto the nozzle/ejector unit 512.

The nozzle/ejector unit 512 utilizes the vapor fraction discharged fromthe structure 510 to increase the temperature and pressure of coolantfluid supplied to the primary heat exchanger 514, as described infurther detail herein. The higher temperature and pressure coolant fluiddischarged from the nozzle/ejector unit 512 flows through the primaryheat exchanger 514 where heat transfer to ambient occurs, therebyreducing the temperature and pressure of the coolant fluid. The splittervalve 518 splits the stream exiting the primary heat exchanger 514 intoa first liquid stream supplied to the secondary heat exchanger 516 and asecond liquid stream routed toward the heat source 512. The splittervalve 518 also reduces the pressure of the second liquid stream. Thesecondary heat exchanger 516 functions to reduce the temperature of theliquid coolant delivered to the liquid inlet of the nozzle/ejector unit512 to a value below the vaporization temperature of the coolant and thesecond pump 528 pumps the second liquid stream at an increased pressureto the nozzle/ejector unit 512.

The pressure regulator valve 520 functions to reduce the high pressureliquid coolant discharged from the mixer 522 to the heat source inletpressure for mixing with the liquid fraction in the mixer 522. The mixer522 combines the liquid coolant flowing from the primary heat exchanger514 with the liquid coolant from the vapor separator 524. The smalllow-power return pump 526 delivers the coolant recycled from vaporseparator 524 to the mixer 522. The outlet of the mixer 522 is deliveredto the structure 510. The first pump 526 can also be used duringstart-up of the cooling system 500.

For cooling, the cooling system 500 would function similarly asdescribed herein but at lower temperature values. With reference toTable D, below, characteristics for the various stages are provided,having associated position blocks.

TABLE D FIG. 8 POSITION PRESSURE MASS FLUX BLOCK TEMP (° C.) (kPa)(g/sec) 550 20 13.7 28.3 560 20 13.0 28.3 570 20 13.0 7.1 580 20 13.021.2 590 41 1092.0 403.7 600 30 1037.4 403.7 610 30 1037.4 382.5 620 301037.4 21.2 630 15 985.5 382.5 640 20 13.7 21.2 650 20 13.7 7.1

The structure 510 would preferably be controlled to a desiredtemperature of approximately 20° C. with an outside ambient temperaturewithin an approximate range of 30° C. to 42° C. (this may vary dependingupon geographic location and season). The low temperature source heatsink could be provided as the ground water source located within theearth below the structure, generally at a temperature of approximately 7to 10° C. Thus, for the structure 510 cooling example, the intermediatetemperature source would be provided as the structure 510, the lowtemperature source as the ground water, and the high temperature heatsink would be ambient air.

To perform the heating function, the cooling system 500 would functionsimilarly as that described for the cooling systems 10, 30, 50, again atlower temperature values. With reference to Table E, below,characteristics for the various stages are provided, having the positionblocks described above for cooling.

TABLE E FIG. 9 POSITION TEMP PRESSURE MASS FLUX BLOCK (° C.) (kPa)(g/sec) 550 30 10.4 112.6 560 30 9.9 112.6 570 30 9.9 28.2 580 30 9.984.5 590 41 1118.4 929.1 600 30 1062.5 929.1 610 30 1062.5 844.6 620 301062.5 84.5 630 −5 1009.3 844.6 640 30 10.4 84.5 650 30 10.4 28.2

For example, the structure would be controlled to a desired temperatureof approximately 25° C. with an outside ambient temperature within anapproximate range of −10° C. to 5° C. (this may vary depending upongeographic location and season). The intermediate temperature sourcewould be provided as the ground water source located within the earthbelow the structure, generally at a temperature of approximately 7 to10° C. Thus, for the structure heating case, the high temperature sourcewould be provided as the inside ambient of the structure, theintermediate temperature source as the ground water and the lowtemperature source as the outside ambient.

From the foregoing description it should be clear that the supersonicvapor compression and heat cycle disclosed for use in efficientlycooling a heat source, such as a fuel cell system, is a significantadvancement which will provide improvements in terms of system operatingefficiency and complexity. Since the system uses waste heat to drive theflow and the temperature increase, pumping power requirements areeliminated, or significantly reduced, thereby increasing the usefuloperating efficiency of the fuel cell system. Moreover, the system ofthe present invention replaces a significant portion of the heatexchanger area with a relatively small nozzle/ejector unit. This permitssmaller coolant mass flow rates which, in turn, permits use of smallerplumbing lines. Vaporization of the coolant at the fuel cell stackoperating temperature allows the entire stack to be maintained at itsoptimum temperature, thereby increasing stack performance. Thesupersonic vapor compression and heat cycle of the present invention canbe used as a cooling system for extracting waste heat from hightemperature heat sources other than fuel cell stacks. Possibleapplications include cooling of vehicle cabins, building interiors, andinternal combustion engines. It is to be understood that the coolantcharacteristic values set forth in the tables are provided for exemplarypurposes only and that the values will vary based on the specificoperating characteristics of the heat transfer system to which thepresent invention is applied.

While the invention has been disclosed primarily in terms of specificembodiments, it is not intended to be limited thereto but rather only tothe extent set forth in the following claims.

What is claimed is:
 1. A fuel cell cooling system comprising: a fuelcell having a coolant path; a first heat exchanger; a first fluidpathway providing fluid communication between an outlet of said fuelcell coolant path and an inlet to said first heat exchanger; a secondfluid pathway providing fluid communication between an outlet of saidfirst heat exchanger and an inlet of said fuel cell coolant path; aliquid coolant flowing through said first and second fluid pathways suchthat a portion of said liquid coolant flowing through said fuel cellcoolant path vaporizes as a result of heat transferred from said fuelcell; a separator disposed in said first fluid pathway for separating avapor stream from said liquid coolant discharged from said fuel cellcoolant path; and a nozzle-ejector unit having a vapor inlet receivingsaid vapor stream, a liquid inlet receiving a liquid stream from one ofsaid first and second fluid pathways, a vapor nozzle for acceleratingsaid vapor stream, a liquid nozzle for directing said liquid stream intocontact with said vapor stream, an ejector through which said mixture ofsaid liquid stream and said vapor stream flows for causing compressionof said vapor, and an outlet for delivering a resulting hightemperature, high pressure liquid coolant to said inlet of said firstheat exchanger for removing heat therefrom.
 2. The fuel cell coolingsystem of claim 1 further comprising a splitter valve disposed in saidsecond fluid pathway for splitting said liquid stream from said secondfluid pathway downstream from said first heat exchanger.
 3. The fuelcell cooling system of claim 2 further comprising a downstream heatexchanger having an inlet receiving said liquid stream from saidsplitter valve for cooling said liquid stream and an outlet in fluidcommunication with said liquid inlet of said nozzle-ejector unit.
 4. Thefuel cell cooling system of claim 1 further comprising an upstream heatremoving heat exchanger having an inlet in fluid communication with saidoutlet of said nozzle-ejector unit and an outlet in fluid communicationwith said inlet of said first heat exchanger.
 5. The fuel cell coolingsystem of claim 4 wherein heat rejected from said upstream heatexchanger is used to heat said vapor stream prior to delivery of saidvapor stream to said vapor inlet of said nozzle-ejector unit.
 6. Thefuel cell cooling system of claim 4 further comprising a pressureregulator valve disposed in said first fluid pathway between said outletof said upstream heat exchanger and said inlet of said first heatexchanger.
 7. The fuel cell cooling system of claim 1 further comprisinga pressure regulator valve disposed in said second fluid pathway betweensaid first heat exchanger and said fuel cell.
 8. The fuel cell coolingsystem of claim 1 further comprising an additional heat removing heatexchanger having an inlet in fluid communication with an outlet of saidseparator and an outlet in fluid communication with said liquid inlet ofsaid nozzle-ejector unit.
 9. The fuel cell cooling system of claim 1further comprising a pump disposed in said second fluid pathway betweensaid first heat exchanger and said fuel cell.
 10. The fuel cell coolingsystem of claim 1 wherein a liquid stream from said separator in saidfirst fluid pathway is supplied to said inlet of said fuel cell coolantpath.
 11. The fuel cell cooling system of claim 1 wherein saidnozzle-ejector unit comprises: a valve body defining a vapor chamber influid communication with said vapor inlet, an expansion chamber, and anacceleration chamber; and a nozzle member extending into said valve bodyand including a central flow passage having a first end in fluidcommunication with said liquid inlet and a second end defining saidliquid nozzle, said liquid nozzle is oriented to discharge a liquid jetinto said acceleration chamber of said valve body; said vapor nozzle isdefined by a restricted area formed between said valve body and saidnozzle member, said vapor nozzle is located between said vapor chamberand said expansion chamber such that said vapor stream is drawn throughsaid vapor nozzle from said vapor chamber and accelerated to supersonicvelocity due to expansion of said vapor in said expansion chamber. 12.The fuel cell cooling system of claim 9 wherein said vapor streamsurrounds and impinges on said liquid jet and acts to accelerate saidliquid jet.